The present invention relates to optical transmission equipment and an optical add-drop multiplexer (OADM) that use a wavelength multiplexing technique. More particularly, the invention relates to optical transmission equipment and an optical add-drop multiplexer that have excellent maintainability.
With the increasing capacity of data system communication represented by the Internet technology, a rapid growth in data volume and an increase in the associated transmission capacity are expected in optical transmission systems. In order to meet such demands, the wavelength multiplexing technique is applied to perform communication by bundling plural signal lights of different wavelengths through one optical fiber. Recently, there is being established a communication network using optical add-drop multiplexers that can drop and add optical signals for each wavelength at plural locations, in addition to transmit large volume of data between two separate sites by means of the wavelength multiplexing technique.
FIG. 1 shows a block diagram of an optical transmission network using optical add-drop multiplexers. An optical transmission network 1000 includes six optical add-drop multiplexers 101 that are connected in a ring configuration through an optical fiber transmission line 102. The optical add-drop multiplexer 101 selects whether to add and drop plural optical signals of different wavelengths for each wavelength, or whether to transmit the optical signals through the multiplexer. In FIG. 1, there are shown the start points and end points of five optical signals λ1 (lambda 1) to λ5 each having different wavelengths, all of which are added and dropped at nodes that can be freely selected.
FIG. 2 is a block diagram showing a principal part of the optical transmission network. FIG. 2 shows the device configuration of a portion of the optical network shown in FIG. 1, from an optical add-drop multiplexer 101-1, to an optical add-drop multiplexer 101-2, and to an optical add-drop multiplexer 101-3. However, in FIG. 2, there are only shown an East side function part for the optical add-drop multiplexer 101-1 and a West side function part for the optical add-drop multiplexer 101-3. The optical add-drop multiplexer 101 includes an optical amplification function part (West) 202-2, an optical amplification function part (East) 202-1, an optical add-drop function part (West) 201-2, and an optical add-drop function part (East) 202-1. The optical amplification function part 202 includes: a reception optical amplifier 203 for amplifying an input optical signal from the optical fiber transmission line 102 and transmitting the amplified signal to the optical add-drop function part 201; and a transmission optical amplifier 204 for amplifying an input optical signal from the optical add-drop function part 201 and transmitting the amplified signal to the optical fiber transmission line 102. The optical add-drop function part 201 includes: an optical drop part having an optical coupler 206-2 and an optical demultiplexer 207; and an optical transmission/add selection part having the optical demultiplexer 207, an optical multiplexer 208, an optical switch 209, and a variable optical attenuator (VOA) 210.
Taking an example of the optical signal flow in a direction from West to East in the optical add-drop multiplexer 101-2, the operation of the entire optical add-drop multiplexer will be described. A received optical signal from the optical add-drop multiplexer 101-1 is amplified by the reception optical amplifier 203 of the optical amplification function part (West) 202-1 of the optical add-drop multiplexer 101-2. Then the amplified signal is transmitted to the optical add-drop function part (West) 201-2. Incidentally, the operations of an optical coupler 206-1 and a laser safety part 205 will be described below with reference to FIG. 4. In the optical add-drop function part (West) 201-2, the optical signal is split into two halves by the optical coupler 206-2, one of which is further branched into lights at each wavelength by the optical demultiplexer 207 and is output from a drop optical port 260-2. The other optical signal is transmitted as it is to the optical add-drop function part (East) 201-1 through an optical fiber 211 connecting the optical add-drop function parts. In the optical add-drop function part (East) 201-1, the optical signal is branched into optical signals at different wavelengths by the optical demultiplexer 207, and the signals are input to the optical switch 209. The optical switch 209 selects and outputs either the transmitted optical signal from West or the added optical signal from an add optical port 250-2 of the optical add-drop multiplexer 101-2. The variable optical attenuator 210 is provided in the later stage of the optical switch 209 to equally adjust all the optical power levels of each of the wavelengths. The light whose optical power levels are adjusted by the variable optical attenuator 210 is wavelength multiplexed by the optical multiplexer 208, and is transmitted to the optical amplification function part (East) 202-1. In the optical amplification function part (East) 202-1, the wavelength multiplexed light is amplified by the transmission optical amplifier 204 and is transmitted to the optical fiber transmission line 102.
FIG. 3 is a view illustrating the optical signal flow from the optical add-drop multiplexer 101-1, to the optical add-drop multiplexer 101-2, and to the optical add-drop multiplexer 101-3 in the optical network of FIG. 1. The optical signal λ1 is dropped and added in the optical add-drop function part (East) 201-1 of the optical add-drop multiplexer 101-1 and in the optical add-drop function part (West) 201-2 of the optical add-drop multiplexer 101-3, while being transmitted through the optical add-drop multiplexer 101-2. Similarly, the optical signal λ2 is dropped and added in the optical add-drop multiplexers A, B. The optical signal λ3 is transmitted through the optical add-drop multiplexer 101-1, while being dropped and added in the optical add-drop multiplexer 101-2. The optical signal λ4 is dropped and added in the optical add-drop multiplexer 101-2, while being transmitted through the optical add-drop multiplexer 101-3.
In FIG. 2, the reception optical amplifier 203 of each of the optical amplification function parts 202-1, 202-2 has a function of compensating the optical power reduction including not only a loss in the optical fiber transmission line but also a loss in the optical add-drop function part. Consequently the optical power level is high. Assuming that the optical power level for one wavelength is +6 dBm in the reception optical amplifier 203, the optical power level for 40 wavelengths reaches +22 dBm which corresponds to a laser standard class 3B defined by JIS C 6082. There is a risk that the eyes will remain damaged by directly seeing such a laser beam. In order to avoid such a risk, the reception optical amplifier 203 includes a laser safety function for automatically reducing the optical power level to about an optical power level at one wavelength (about +5 dBm or less) which corresponds to a class 1 standard, by detecting an output open of the optical fiber by reflected light. In FIG. 2, the laser safety function is realized using the optical coupler 206-1 and the laser safety part 205. The laser safety function in the optical amplifier as described above is disclosed in JP-A No. 200130/1997, JP-A No. 144687/2001, and JP-A No. 335214/2002.
In the configuration of FIG. 2, it is assumed that a failure occurs in the optical add-drop function part (East) 201-1 of the optical add-drop multiplexer 101-2 and the relevant function part is needed to be replaced. In this case, main signal interruption occurs in the two signals λ1, λ4 of the optical signals shown in FIG. 3, and main signal interruption should not occur in the optical signals λ2, λ3 that are originally not involved in the replacement. However, when the optical fiber 211 connected between the optical add-drop function part (West) 201-2 and the optical add-drop function part (East) 201-1 is removed, the laser safety part 205 detects an output open of the optical fiber by reflected light, thereby providing laser safety to the reception optical amplifier 203. Given the optical level per wavelength of +6 dBm in the output of the reception optical amplifier 203, the reception optical amplifier 203 of the optical amplification function part (West) 202-2 amplifies the three signals of λ1, λ2, λ3, so that the total optical power level of all the optical signals is +10.8 dBm. This will be reduced to +5 dBm because the laser safety functions due to removal of the optical fiber connected between the optical add-drop function parts. In other words, the optical power level per wavelength is reduced by 4.8 dB to +1.2 dBm, which has an impact on main signal continuity of the optical signals λ2 and λ3, causing main signal interruption. The above description has been made on the optical signals of three wavelengths. However, assuming that the present system is a system supporting 40 wavelengths, the optical level is reduced by up to 16 dB according to the calculation in the same way as described above.
The simplest way to solve the above problem is to insert an optical isolator into an input end from the reception optical amplifier of the optical add-drop function part. In this case, however, the laser safety does not function because the optical fiber is opened during the replacement of the optical add-drop function part. From the output of the reception optical amplifier to the fiber connected between the optical add-drop function parts, the optical loss occurs only in the optical isolator and the optical coupler, and their loss is at most about 2 dB in total. In the case of 40-wavelength system, the eyes may be damaged by directly seeing a maximum of +20 dBm during removal of the optical fiber 211 between the optical add-drop function parts. For this reason, in the method of inserting the optical isolator, it is necessary to have a structure in which a light blocking function such as an optical fiber connection shutter is provided to prevent the eyes from directly seeing the light from the optical fiber.